This invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device having a thick alignment film and good electrooptical characteristics.
Heretofore, it has been proposed to provide an alignment film on a substrate by coating the surface of a substrate with a silane coupling agent, etc. for aligning liquid crystal homeotropically to the surface of the substrate. As the silane coupling agent, there have been proposed alkyltrialkoxysilanes (Japanese Laid-open Patent Application No. 50947/75), alkyltrialkoxysilanes and tetraalkoxysilanes (Japanese Laid-open Patent Application No. 63955/75), haloalkylalkoxysilanes such as fluoroalkoxysilanes (Japanese Laid-open Patent Application Nos. 68355/75 and 36150/75).
Furthermore, there have been proposed alignment films of polysiloxane obtained by hydrolysis and condensation of fluoroalkoxysilane (Japanese Laid-open Patent Application No. 68355/75) and alignment films of polysiloxane such as silicone oil (Japanese Laid-open Patent Application No. 94693/73).
The alignment film on the substrate for a liquid crystal display device has a function as an insulation film when a "NESA" (a trademark) film (transparent electro conductive film) is provided directly under the alignment film, and thus a thin alignment film undergoes electrochemical reaction between a very small amount of impurities, particularly ionized impurities in the liquid crystal and the NESA film during the functioning of a liquid crystal display device, resulting in deterioration. Furthermore, the NESA film on the substrate plate is liable to make patterns of the NESA film visible even if not displayed, when liquid crystal is filled in the liquid crystal display device. Thus, it is necessary that the alignment film has an appropriate thickness, for example, 300-2,000 .ANG., also for the prevention of the patterns of NESA film from being visible when not displayed.
When the substrate for a liquid crystal display device is coated with a silane monomer such as the above-mentioned alkylsilane, etc., only a very thin alignment film, for example, a film having a thickness of less than 100 .ANG., is obtained. When the substrate is coated with the above-mentioned polysiloxane, and the thickness is increased, cracks develop on the alignment film with poor adhesiveness of polysiloxane to the substrate, because polysiloxane has no groups capable of contributing to condensation reaction between the substrate and polysiloxane molecules, or between polysiloxane molecules themselves when cured. Thus, the alignment films obtained by coating of silane monomers or polysiloxane have had problem in their practice.